My DS goes to a Music with Mummy, with his grandad, on the day his grandparents look after him. The woman who runs it does lots of classes in the 1-3yr age group. AFAIK they follow a standard MWM structure, with a hello song, theme of the term, listening slot, instrument slot, moving and action songs, goodbye song. They then have 30mins for tea / coffee / biscuit and she gets a big toy box out. She has a couple of kids herself, is v. musical and also has a PGCE. I've only been a couple of times, but I think that the programme's pretty good, but the sessions are really good because she's so good. A lot of it is about managing a group of toddlers! I love the tea / buscuit / play time, as a chance to get talking to people.
She runs 8 sessions per week in the same age group, and says that's more worth her while than doing different age groups as she has to pay extra per age group not per session.
The other franchise that seems to be widepread is "Monkey Music" - most of my friends in different areas go to those classes.
We are also going to start going to a singing session run by an individual through a church (i.e. not a franchise). I don't know whether individual groups would be any better / worse than franchises. Personally, if I were to do it, I'd like the franchise providing all the programme, and also the increased advertising (but I'm tone deaf, so I couldn't do it!).
Looks like a pretty good way to make some money - when your own kids are pre-school you can take them with you, and as it's term time only for when they are at nursery / school. I don't know what the franchise costs, or what we pay (PILs pay, I think it works out as £3.50 per session, paid half-termly), but I once worked out that she seems to get a pretty decent amount per session (upto about 12 kids) if she doesn't have to pay too much for the franchise.
She used to use her own home till parking became a problem, and now rents a small room in a village hall.